xattr: extract simple_xattr code from tmpfs

Extract in-memory xattr APIs from tmpfs. Will be used by cgroup.

$ size vmlinux.o
   text    data     bss     dec     hex filename
4658782  880729 5195032 10734543         a3cbcf vmlinux.o
$ size vmlinux.o
   text    data     bss     dec     hex filename
4658957  880729 5195032 10734718         a3cc7e vmlinux.o

v7:
- checkpatch warnings fixed
- Implement the changes requested by Hugh Dickins:
	- make simple_xattrs_init and simple_xattrs_free inline
	- get rid of locking and list reinitialization in simple_xattrs_free,
	  they're not needed
v6:
- no changes
v5:
- no changes
v4:
- move simple_xattrs_free() to fs/xattr.c
v3:
- in kmem_xattrs_free(), reinitialize the list
- use simple_xattr_* prefix
- introduce simple_xattr_add() to prevent direct list usage

Original-patch-by: Li Zefan <lizefan@huawei.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Hillf Danton <dhillf@gmail.com>
Cc: Lennart Poettering <lpoetter@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/shmem_fs.h

@@ -5,6 +5,7 @@
 #include <linux/mempolicy.h>
 #include <linux/pagemap.h>
 #include <linux/percpu_counter.h>
+#include <linux/xattr.h>
 
 /* inode in-kernel data */
 
@@ -18,7 +19,7 @@ struct shmem_inode_info {
 	};
 	struct shared_policy	policy;		/* NUMA memory alloc policy */
 	struct list_head	swaplist;	/* chain of maybes on swap */
-	struct list_head	xattr_list;	/* list of shmem_xattr */
+	struct simple_xattrs	xattrs;		/* list of xattrs */
 	struct inode		vfs_inode;
 };